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GLP-1 Muscle Loss: How Much Lean Mass You Lose and How a DEXA Scan Tracks It

Weight loss on a GLP-1 medication such as Ozempic, Wegovy or Zepbound includes some lean mass. Here is what the trial DEXA scans found, what "lean mass" really measures, and how a baseline and repeat DEXA scan show whether your own loss is fat or muscle.

Published

October 29, 2026

Read time

9 min read

Medically reviewed by

Rhonda Collins, FNP-C

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Some of the weight you lose on a GLP-1 medication is lean mass, and a DEXA scan is how you see how much. In the trial body composition substudies, lean mass made up roughly a quarter to two fifths of the weight lost on semaglutide (the drug in Ozempic and Wegovy) and tirzepatide (the drug in Zepbound and Mounjaro) ^[1][2][3]. That lean mass is not all muscle: it also includes organs, water and connective tissue ^[4]. And losing weight by eating less costs some lean mass whatever the method: in the tirzepatide trial, the placebo group lost lean mass in the same proportion ^[2].

A bathroom scale only shows total weight. A DEXA body composition scan splits that weight into fat, lean tissue and bone, so a baseline scan and a repeat scan on the same machine show how much of your own loss was fat.

This guide covers the muscle and body composition question. For how GLP-1 medications work and their wider benefits and risks, see GLP-1 benefits and risks explained. For the blood tests often used during treatment, see the GLP-1 medications and monitoring guide.

How much muscle do you lose on a GLP-1?

The best numbers come from the DEXA substudies inside the large weight-loss trials, where a subset of participants had a whole-body DEXA scan at the start and at the end.

Semaglutide (STEP 1). In the 140-adult DEXA substudy, people on semaglutide lost 15.0% of their body weight over 68 weeks, 19.3% of their total fat mass and 27.4% of their visceral fat ^[1]. Their lean mass fell by 9.7%. Because fat fell faster, lean mass as a share of body weight rose by 3.0 percentage points, and the authors reported no major body composition changes with placebo. Working from the trial’s supplementary appendix, a 2025 review in Obesity Reviews calculated that lean mass fell by an average of 6.92 kg on semaglutide, about 40% of the weight lost ^[3].

Tirzepatide (SURMOUNT-1). In the 160-adult DEXA substudy, people on tirzepatide lost 21.3% of their body weight over 72 weeks, 33.9% of their fat mass and 10.9% of their lean mass ^[2]. About 75% of the weight lost was fat and 25% was lean mass. That split was the same in the placebo group, who lost less weight through diet and activity alone, and it held across most subgroups by sex, age and amount of weight lost.

Read the two together with care. These are separate trials, not a head-to-head comparison, and each substudy scanned only a small part of the full trial. Across GLP-1 studies, a 2024 review found lean mass ranging from about 15% or less to 40% to 60% of the weight lost, and linked that spread to differences in population, drug and other health conditions ^[4]. Both substudies were funded by the drugs’ manufacturers.

Is lean mass the same as muscle?

No. DEXA reports lean mass as everything that is not fat or bone. That includes skeletal muscle, but also the liver, heart and other organs, body water and the water held in fat tissue ^[4]. Muscle usually accounts for about half of lean mass ^[3].

So a lean mass drop overstates the muscle drop. Working from the STEP 1 figures, the Obesity Reviews authors gave a conservative estimate that people on semaglutide lost at least 10% of their muscle mass over 68 weeks, assuming muscle made up at least half of the lean mass lost ^[3]. In obese mice, GLP-1 medicines reduced liver mass by about 20% while skeletal muscle changed less, a sign that organs account for part of the lean loss ^[5]. That finding is from mice and may not carry over to people.

Does GLP-1 weight loss make you weaker?

The evidence so far suggests muscle loss on a GLP-1 is mostly in line with the amount of weight lost, though strength has been studied far less than mass.

• Muscle quality may improve. A 2024 review of magnetic resonance imaging (MRI) studies concluded that muscle changes on GLP-1 medicines appear to be adaptive: muscle volume fell about as much as expected for the weight lost, age and health status, while fat inside the muscle fell ^[4]. The authors noted that older age and more severe illness may raise the risk of sarcopenia, meaning low muscle mass and strength.
• Strength held in a small trial. In a 12-week proof-of-concept trial of semaglutide in adults with obesity and type 2 diabetes, about 70% of the weight lost was fat and 30% lean mass. The thigh muscle got smaller, but neither leg strength nor hand grip strength decreased ^[5]. The trial was small and short.
• DEXA does not measure strength. A DEXA scan shows how much lean tissue you have and where it is. It does not show how strong that muscle is or how well it works. The Obesity Reviews authors called for studies that measure muscle function, mobility and strength, not mass alone ^[3].

How a DEXA scan tracks fat and lean mass on a GLP-1

DEXA passes two low-dose X-ray beams over you while you lie on an open table. Fat, lean tissue and bone absorb them differently, so the scan reports fat mass, lean mass and bone for each arm, each leg and the trunk, plus a visceral fat estimate. Mito’s DEXA body composition guide explains the method and its accuracy in detail.

On a GLP-1, the value is in comparing scans over time:

1. Get a baseline. A scan before you start, or early in treatment, gives you fat mass, lean mass and visceral fat to compare against.
2. Repeat after a real change. Compare the change in fat mass with the change in lean mass. If most of the weight you lost came from fat, your lean mass share goes up even though your lean mass in kilograms goes down, as it did on average in STEP 1 ^[1].
3. Look at the regions. Arm and leg lean mass is mostly skeletal muscle, so a falling arm and leg lean mass tells you more about muscle than total lean mass, which includes the organs in your trunk.

How often to repeat it. There is no single recommended interval for body composition scans. Repeat a scan when the change you expect is larger than the scanner’s measurement error. The International Society for Clinical Densitometry (ISCD) says each DEXA facility should measure its own precision and least significant change, meaning the smallest difference between two scans that you can trust as real. It sets the minimum acceptable precision for a DEXA technologist at 3% for total fat mass and 2% for total lean mass ^[6]. A lean mass change smaller than that on a rescan may be noise.

How to get comparable repeat DEXA scans

DEXA counts body water as lean mass, so anything that shifts your water can shift your lean mass number. That matters on a GLP-1, when how much you eat and drink may change from week to week.

• Use the same machine and location. Results from different equipment cannot be compared directly, so RadiologyInfo advises follow-up DEXA scans at the same facility, ideally on the same machine ^[7].
• Keep the same time of day and the same state of food and fluid. In 32 athletic men, eating and drinking normally before a scan produced only small changes in DEXA fat-free mass, and the authors found DEXA acceptable when recent food and fluid stayed below about 500 g ^[8].
• Skip hard exercise, saunas and steam rooms for 12 hours beforehand, as Mito’s DEXA scan instructions ask. Food and drink rules depend on the provider that scans you, and the DEXA scan page lists them.
• Keep supplements and routines steady between scans where you can, and note anything that changed, so you can read a lean mass shift in context.

For reading the report itself, including the whole-body bone result, see how to read DEXA scan results.

What studies tested to limit lean mass loss

The research on protecting muscle during weight loss centers on resistance training and protein. Several systematic reviews and meta-analyses found that resistance training was effective for maintaining muscle mass during calorie restriction in people with overweight or obesity ^[3]. With the reduced appetite of GLP-1 treatment, many people may eat too little protein, and the same 2025 review recommended that protein intake be monitored ^[3]. Its authors concluded that people taking these medications for obesity should be in a comprehensive program that emphasizes adequate protein, micronutrients and resistance training ^[3].

These are findings and expert recommendations, not a plan for you. If a repeat scan shows lean mass falling faster than you expected, bring the report to the clinician who prescribes your medication, who can weigh it against your age, health and goals.

Pair your DEXA results with blood work

A DEXA scan shows what you are losing. Blood tests show how your metabolism is responding. The GLP-1 monitoring guide covers markers such as HbA1c and the comprehensive metabolic panel, and the GLP-1 nutrient deficiencies guide explains when to check vitamin D, vitamin B12 and ferritin while you are eating less.

Get a DEXA body composition scan through Mito

Mito offers a DEXA body composition scan with no referral needed. Depending on your state, it is booked through Fitnescity Health at a partner imaging center, hospital or wellness facility, or at a BodySpec studio in California, Colorado, Illinois, Massachusetts, Texas or Washington. It is not done during pregnancy or soon after a barium or contrast exam, and the product page shows your provider, price, preparation and how to book. Your report lands in your Mito dashboard, next to your lab results. Rescan at the same location so your numbers stay comparable.

References

1. Wilding et al., 2021: Impact of semaglutide on body composition in adults with overweight or obesity, exploratory analysis of the STEP 1 study. doi:10.1210/jendso/bvab048.030
2. Look et al., 2025: Body composition changes during weight reduction with tirzepatide in the SURMOUNT-1 study. doi:10.1111/dom.16275
3. Mechanick et al., 2025: Strategies for minimizing muscle loss during use of incretin-mimetic drugs for treatment of obesity, Obesity Reviews. doi:10.1111/obr.13841
4. Neeland et al., 2024: Changes in lean body mass with glucagon-like peptide-1-based therapies and mitigation strategies. doi:10.1111/dom.15728
5. Langer et al., 2026: Weight loss with GLP-1 medicines does not result in a disproportionate loss of muscle mass or function in obese mice and humans. doi:10.1016/j.xcrm.2026.102665
6. International Society for Clinical Densitometry: Adult Official Positions
7. RadiologyInfo.org: Bone Densitometry (DEXA, DXA)
8. Food, fluid and time-of-day effects on DEXA and bioimpedance spectroscopy in 32 athletic men. doi:10.1017/S0007114517000551

Frequently asked questions

How much muscle do you lose on Ozempic or Wegovy?

In the STEP 1 trial of semaglutide, the drug in Ozempic and Wegovy, a DEXA substudy of 140 adults found that lean mass fell by 9.7% over 68 weeks while fat mass fell by 19.3%. A later review calculated that lean mass made up about 40% of the weight lost. Lean mass includes organs and water as well as muscle, so muscle loss itself was smaller than that figure.

Does tirzepatide (Zepbound) cause muscle loss?

Some. In a DEXA substudy of 160 adults in the SURMOUNT-1 trial, people on tirzepatide lost 10.9% of their lean mass and 33.9% of their fat mass over 72 weeks. About 75% of the weight lost was fat and 25% was lean mass, the same split as in the placebo group, who lost less weight through diet and activity alone.

Is muscle loss on a GLP-1 worse than with dieting?

Not clearly. Losing weight by eating less always costs some lean mass. In SURMOUNT-1, the share of weight lost as lean mass was about 25% with both tirzepatide and placebo. A 2024 review found lean mass ranged from about 15% to 60% of weight lost across GLP-1 studies, depending on the population, the drug and how lean mass was measured.

How can I tell if I am losing muscle or fat on a GLP-1?

A bathroom scale cannot tell you. A DEXA body composition scan separates fat mass, lean mass and bone, so a scan before or early in treatment and a repeat scan later show how much of the weight you lost was fat. Use the same machine and the same preparation each time so the comparison reflects your body rather than the scanner or your hydration.

How often should you get a DEXA scan on a GLP-1?

There is no single recommended interval. Repeat the scan when the change you expect is larger than the scanner's measurement error, for example after a meaningful amount of weight loss, rather than on a fixed weekly or monthly schedule.

Can you build or keep muscle while taking a GLP-1?

Several systematic reviews found that resistance training helped maintain muscle mass during calorie restriction in people with overweight or obesity, and a 2025 expert review called for resistance training and adequate protein alongside GLP-1 treatment. How that applies to you is a question for the clinician who prescribes your medication.